MENDEL/SU AM) MUTATION 



345 



The 2$-chromosome Mutants (gigas group). In the mutanl tonus of 

 tin's group, of which (Enothera gigas is :i member, the somatic number of 

 chromosomes is 28 ra1 her 1 lian I 1 ; t he plants are te1 raploid I Fig. 135, B). 

 How this condition arises is not certainly known. Stomps (1912 

 believed it to he the result of 1 he union of 1 wo unreduced gametes, whereas 

 Gates (19096) suggested thai 'the doubling in the chromosome number 

 had probably occurred as the result of a suspended mitosis in i he feii ilized 

 egg or in an early division of the young embryo." Strasburger (19106) 

 also adopted the latter view. 







*»y e 



Fk;. L35. — Chromosomes in (Enothera mutants. 



A, interkinesis in (E. Lamarck i ana; 7 split chromosomes. />', same in (E. gigas; 11 

 split chromosomes. C, somatic cell of CE. semilata; 1") chromosomes. I), tnetaphase of 

 homceotypic mitosis in CE. biennis lata, showing 8 chromosomes on one spindle and 7 on 

 the other. Spores and gametes with these numbers will result. E, the 21 chromosomes 

 in a mutant from (E. Lamarckiana. (A and Ii after Davis, 1911; C and I) after Gates ami 

 Thomas, 1914; E after Lute, 1912.) 



The mutants of the gigas group are characterized chiefly by an unusu- 

 ally large size, not only of the plant as a whole but also of its anatomical 

 eoustit uents. In t he tet raploid mutant (Enothera stenona res, tot instance, 

 Tupper and Bartletl (1916) found that the change from the diploid to 



the tetraploid condition is concomitant with a 50 per cent increase in 

 the length of the vessel, a 150 per cent increase in t he area of its cross 



section, a 50 per cent increase in the length and diameter of the tracheids, 

 an increase in t he t hree dimensions of t he medullary ray cells, and a break- 

 ing up of the tall multiple ray into a number of thin simple cays. 



